The present invention is directed to a method and apparatus and rejuvenating drilling mud from an oil, gas or other similar well and form similarly rejuvenating other slurries containing solid impurities.
In the drilling of oil wells, drilling mud is forced down the drill string, emerging therefrom at the drill bit, and then passes upwardly on the outside of the drill string between the drill string and the wall forming the bore hole. The drilling mud which is frequently referred to as "drilling fluid" comprises a slurry formed of a liquid carrier such as water as diesel fuel and usually includes solid additives such as extremely fine barite particles of less than 74 microns in size. The drilling mud functions to carry the cuttings to the surface, to control the pressure in the formation being drilled, to lubricate the drill string and the bit and to avoid sidewall collapse of the bore hole. Various additives are supplied to the drilling mud to improve its performance; since the additives have substantial value, it is desirable that their loss be avoided. Rejuvenation of drilling mud includes the removal of the earth cuttings. The terms "solids" and "cuttings"as used hereinafter refer to stone or earth particles cut from the earth by the drilling bit.
It is well known that the presence of cuttings in the drilling mud has such harmful effects as decreasing the life of pumps and other expensive equipment. However, removal of the cuttings with a substantial portion of expensive prior known equipment normally results in the loss of additives, such as barite. It is also desirable that substantially all of the entrained gases and air be removed, and that the drilling mud not be diluted. At the present time, it is conventional to remove cuttings by such devices as shale shakers, desanders, desilters, and centrifuges. Degassers are also utilized when needed.
Thus, the drilling fluid must be maintained in good physical and chemical condition in order to be able to perform its intended functions. The cuttings range in size from large pieces (3/4 in. diam.) to very fine particles. The large cuttings, if permitted to be circulated in the fluid down the drillpipe, would plug up the rock bit channels and the drill string tools. Removal of the large cuttings has been previously accomplished by passing the drilling fluid stream over a coarse vibrating screen (shale shaker) where the large pieces are screened out. Unfortunately, the vibratory action of the shale shaker causes many high pressure forces to be applied to the cuttings particles causing them to be reduced in size so as to render them more difficult to separate from the drilling fluid; thus, while the shale shaker improves the mud by removing the larger cuttings particles, it also has the negative effect of increasing the number of smaller cuttings particles which pass through the screen and remain in the mud. Subsequent passage through hydrocyclones and pumps further shear and degrade the cuttings. The smaller cuttings particles in the drilling fluid are not easily removed and their presence can have very detrimental effects on the entire drilling process. As cuttings content builds up, severe abrasion of the rock bit and drill string tools takes place and the properties of the drilling fluid itself are seriously impaired and tend to become uncontrollable. Moreover, cake deposition of cuttings particles on the side of the hole increases the friction coefficient, the main cause of pressure differential pipe sticking.
One method of reducing the percentage of smaller sized cuttings particles from the mud stream is by dilution of the whole mud volume by adding water or other liquid. This is often impractical and almost always expensive. The costs of adding water, providing for more pit volume, etc., will generally make it more desirable to remove solids mechanically such as by screening. However, dilution is frequently used to reduce the percentage solids content during drilling of the surface hole (the upper or shallower portion of the hole).
In a typical apparatus, the drill mud from the drilling apparatus is passed through six modular units or work stations comprising: (1) a shale shaker; (2) a degasser; (3) a desander; (4) a desilter; (5) a mud cleaner and (6) a centrifuge. The mud is initially fed into a shale shaker which separates large particles with the remaining drilling mud then being delivered to a first tank. It is pumped by a centrifugal pump from the first tank, through a degasser, and enters a second tank from which it is then pumped to a desander, and enters a third tank. It is then pumped through a desilter, or a mud cleaner, which, like the desander, is in the form of a cyclone filter or cleaner. It then enters a fourth tank, and is delivered to the mud tank or mud holding tank from the fourth tank.
Accordingly, the mud is moved in a batch type operation from one module (unit) of equipment to the next module by means of at least three energy consuming centrifugal pumps. Since the use of all modules of the equipment is not a continuous operation, only a portion of the total drilling fluids stream is actually processed through the entire system and the untreated mud in effect contaminates the treated mud.
It is generally accepted in the industry that present equipment usages permit no better than 50% removal of the solids (cuttings) of given size from the mud. While some mud systems have multiple units for "better" solids control, the costs are excessive. On a typical deep well where mud costs are high, equipment rental costs on the average are $1,500 per day. Some wells incur equipment rental costs in excess of $2,000 per day. Horsepower requirements for the pump and module operation of the equipment ranges from 430 to 450 Hp. Moreover assembly and piping costs often equal the buy-out price of the equipment. It will consequently be apparent that present known approaches to drilling mud treatment are both functionally inadequate and financially burdensome.
Although there have been proposals for simpler methods and apparatus for treatment of drilling mud such as in Lee U.S. Pat. No. 4,350,591, such apparatus has not found any substantial acceptance in the industry. This apparatus utilizes a filter belt passing over drums, and vibrating plates in engagement with the drilling mud delivered to the filter to assist in forcing the drilling mud through the filter.
The most dramatic improvement in mud treatment is the apparatus and method disclosed in prior U.S. Ser. No. 541,923 filed on Oct. 14, 1983 in which a continuous movable filter belt is mounted on supporting rollers for providing a horizontal flight to the upper surface of which drilling mud is supplied by a unique roller weir assembly. The upper flight of the filter belt passes over a vacuum table which incorporates degasser means for separating gas from the mud which has passed through the filter belt. A significant aspect of the operation of the apparatus is that substantially all of the cuttings are separated from the mud in that only the mud passes through the filter belt while the cuttings remain on the upper surface. Different embodiments of the apparatus incorporate different types of degasser means. The present invention represents an improvement over the aforementioned apparatus and method which provides even better results and which is easier to service and maintain.